In complex-oxide molecular beam epitaxy (MBE) processes, multiple source elements with significantly different oxygen affinities are used together. However, the source oxidation of easily oxidized elements leads to flux instability, and maintaining stable fluxes for all elements in an oxygen environment is a challenging task. If all elements used are easily oxidized, such as in the case of Sr(Ca, Ba)TiO3, a low background oxygen pressure, in the 10−7 Torr range, is sufficient and flux instability is not critical. However, this issue becomes prominent when an easily oxidized element is used together with a difficultly oxidized element, such as Cu in cuprates and Pb in PbTiO3, which require a high pressure (approximately 10−5 Torr) of background ozone to oxidize the difficultly oxidized elements, Cu and Pb. Theis et. al. showed that while the flux rate for a Ti source remained fairly constant at a background ozone pressure of 2×10−6 Torr, it dropped by 2.5% per hour when subjected to an ozone background pressure of 5×10−5 Torr. For elements such as Ba, a greater than 50% flux drop has been observed under similar oxidation conditions. In such a harsh oxidation environment, a real-time flux monitoring scheme, such as atomic absorption spectroscopy (AA), has been employed in order to achieve a flux variation of less than 1% for the more easily oxidized elements over several hours of growth. However, such a scheme increases the complexity of the growth process as the number of elements grows, and it is also cumbersome to implement. It has now been discovered that minimizing the oxygen partial pressure near the source surface, even in a harsh oxygen environment, provides a superior solution to the flux instability problem. When the O2 partial pressure near the source surface is kept negligible, the flux has been found to be stable throughout the entire growth cycle, thus eliminating the need for real-time monitoring.